Off Topic: typedef declares object / enum to int

From: E. Robert Tisdale (E.Robert.Tisdale_at_jpl.nasa.gov)
Date: 02/23/05 

Date: Wed, 23 Feb 2005 12:08:58 -0800

Martin wrote:

> This post asks two questions, which I've illustrated in one C source file
> (see below), which clean compiles on my GNU compiler.
>
> 1. In K&R2, Section A8.9 it says "Declarations whose storage class specifier
> is typedef do not declare object." When I compile and run my sample code,
> the value "2" is displayed. Clearly the typedef has created the enumerators
> S1, S2, and S3. This appears to contradict K&R2.

No!
S1, S2 and S3 are *values* that an object of type an_enum_t can have.
They are *not* themselves objects.

> 2. The program below was created to emulate a Lint diagnostic
> [that] I got in a much larger project.
> In that project, the equivalent line to fn(SYMBOL) gives the warning
>
> Converting enum '{...}' to int
>
> which I find puzzling. Enumerators are int anyway, whereas the enumeration
> has an integral value (according to K&R2, Section A4.4). A warning about an
> enumerator (int) being converted to an int seems meaningless;
> and a warning about the enumeration (integral type) being converted to int
> doesn't make sense because it's the enumerator S3 that gets passed to fn().
> I've reproduced Lint's explanation of the warning below.

> cat main.c
         // typedef declaration; Lint warning
         #include <stdio.h>

         typedef enum { S1, S2, S3 } an_enum_t;

         #define SYMBOL S3

         typedef unsigned int Word;

         void fn(Word w) {
           // deliberately empty
           }

         int main(int argc, char* argv[]) {
            fn(SYMBOL);
            printf("%i\n", SYMBOL);
            return 0; // success
           }

> gcc -Wall -std=c99 -pedantic -o main main.c
> ./main
         2

> This is Lint's description of the warning:
>
> 641 Converting enum to int -- An enumeration type was used in a context that
> required a computation such as an argument to an arithmetic operator or was
> compared with an integral argument. This warning will be suppressed if you
> use the integer model of enumeration (+fie) but you will lose some valuable
> type-checking in doing so. An intermediate policy is to simply turn off this
> warning. Assignment of int to enum will still be caught.
>
> This warning is not issued for a tagless enum without variables. For example
>
> enum {false,true};
>
> This cannot be used as a separate type. PC-lint/FlexeLint recognizes this
> and treats false and true as arithmetic constants.

So your complaint is that lint is nitpicking?

Relevant Pages

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